1. Field of the Invention
The present invention relates to a method for surface treatment of copper or copper alloys (hereinafter collectively referred to as "copper"), which is useful in applications such as the manufacture of printed circuit boards.
2. Description of the Background Art
Up to now, conventional printed circuit boards used extensively in electronic appliances have been built on an FR-4 grade base material, such as a lamination of a copper foil, a prepreg, a circuit board for internal layer, another prepreg, and a copper foil, overlaid in that order. However, due to the rapidly accelerating trend in recent years toward packing numerous electronic components tightly onto a board, thermal resistances that cannot be met by the FR-4 grade materials are being required since such components tend to heat up. Meanwhile, as the frequency of electronic signals passing through a printed circuit board becomes higher and higher, the base material is required to have reduced levels of dielectric loss tangent as well as dielectric constant.
Accordingly, instead of ordinary bisphenol-A resin or novolak type epoxy resin as the impregnating resin for prepregs, use of resins with higher glass transition temperatures such as heat-resistant epoxy resin, bismaleimide-triazine resin, polyphenylene ether, or polyimide is becoming popular for improving the heat resistance and the high frequency properties of a printed circuit board. Further, utilization of these highly heat resistant materials is also increasing as an inter-layer insulating material for manufacturing a printed circuit board via a build-up process. On the other hand, these resins with high heat resistance, i.e. resins having glass transition temperatures (as determined by the TMA method; applicable throughout this specification) of 150.degree. C. or higher, are harder than conventional epoxy resins, and exhibit less adhesion to a copper foil as they possess a fewer number of polar groups. Therefore, when such a material is subjected to severe conditions such as a pressure cooker test where the material is humidified followed by heating, separation occurs between the resin and the copper foil to reveal the shortcomings mentioned.
As a solution to the above problems, for example, a method is disclosed in Japanese Patent Application Laid-Open No. 6035/1994 wherein the surface of a copper foil on which a prepreg is to be applied is first oxidized, then reduced, and treated further with an aminosilane coupling agent. However, since the silane coupling agent applied on the copper surface tends to be washed away in a cleaning process, it cannot be used for a continuous printed circuit board manufacturing process based on a horizontal type conveyor system for which water cleaning is an integral part.
Also, in another method disclosed in Japanese Patent Application Laid-Open No. 266241/1986, the surface of a copper foil to which a prepreg is to be applied is first oxidized, then reduced, and treated further with an aminotriazole solution or the like. However, as pointed out in Japanese Patent Application Laid-Open No. 258870/1995, it has become clear that the aminotriazole treatment is far from beneficial but detrimental to resin adhesion to the copper surface.
The present invention has been made in consideration of the above circumstances and has its objective in providing for a method for bonding a resin which has a high glass transition temperature but low affinity to copper surfaces, with a copper surface with sufficient adhesive strength.